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Are green stink bugs a threat to soybeans?

It’s no secret the increased usage of Bt corn and cotton has dramatically reduced the use of organophosphate and pyrethroid insecticides, nor that this decrease has created an explosion of stinkbug pressure in the Southeast over the past decade.

How to manage these stinky pests has proven to be more challenging than expected. Challenging, not just for production farmers, home gardeners, even homeowners, but likewise challenging for veteran entomologists who are trying to find economically and environmentally sound systems to manage these pests.

Organophosphate and pyrethroid insecticides which were used on virtually every crop grown in the Southeast, are effective in reducing most stink bugs. Subsequently, they were not an economically significant pest on any crop in earlier times.

That led to two distinct problems: The dramatic decline in use of these two once popular insecticide families created a positive environment for stink bugs in a wide range of host plants. And, the lack of stink bug pressure in the decades of the 1980s and1990s meant university researchers and chemical company product developers focused their attention on other more pressing insect problems.

The end result has been an explosion of stink bug pressure across a wide range of crops in the Southeast. How to put the proverbial genie back in the bottle is an ongoing challenge for entomologists across the region.

An increase in soybean acreage in the Southeast may be contributing to the stink bug problem. For certain, soybean growers should scout fields for these pests and treat when necessary.

When necessary is a bit more difficult than expected, according to veteran Virginia Entomologist and IPM Leader Ames Herbert.

In cooperative research with the University of Maryland and the University of Delaware, Herbert is in his third year of research in which they placed stink bug-containing mesh cages about the size of a pillowcase to introduce one insect per foot of row.

For the first two years they placed the insects in soybeans at growth stage R4, R5, R6 and R7. They left the insect in for three weeks, pulled the insects, re-clipped the bag and let the soybean plant go to maturity. The researchers evaluated all the factors that could affect soybean yield and quality.

“In two years (2007 and 2008) at three locations, we saw no significant differences. Some of our colleagues in the Mid-South suggested we needed higher insect populations,” Herbert says.

In 2009, they upped the insect numbers to 0, 1, 2 and 4 stink bugs per foot of row. They did this with both green and brown stink bugs using both nymphs and adults. The complicated test is still being analyzed and will be available soon to growers.

In addition to these tests, Herbert says his research team had an opportunity to work with two growers, which yielded some surprising results.

“We had four trials with growers. We had stink bugs well over threshold levels. We dropped in small plot efficacy trials to evaluate different insecticides to control stink bugs. It dawned on us we had insecticides that would take insect levels to zero and we had check plots where stink bugs were allowed to feed, so we could capitalize on this by getting yield and quality comparisons,” Herbert explains.

In the four tests, they took the worst treatment, which was the control plots and the best insecticide treatment. From the four tests they took the best and worst, carefully transporting two, 3 row-foot sections of soybeans from each test plot back to the lab for analysis.

From these samples they checked plants per foot of row, pods per three foot of row, seed and dry weight. They took a 500 seed sub-sample and checked purple stain, grey mold and any other factor that could be stink bug damage.

“In all four fields non-treated soybeans had stink bug numbers well above threshold levels. In all four fields different insecticides knocked insect numbers to well below threshold for the rest of the season. With that information alone, I would have advised the grower to spray,” the long-time Virginia IPM Leader says.

However, after detailed scientific analysis of the treated beans versus the check plots, there was still no significant difference in terms of yield and quality of treated versus untreated soybeans.

“The results are hard to understand. Our Soybean Specialist, David Holshouser, warned us that 2009 was virtually an ideal growing season for soybeans. Whether we would get the same results, if the beans were under stress — we don’t know,” Herbert says.

Across the Southeast the species of stink bugs in soybeans changes, but threshold levels are generally the same among the states, about 1-2 per foot of row. In the Virginia test, Herbert found threshold levels well above traditional treatment threshold, yet found no significant damage to soybean yield or quality.

In Virginia, green stink bugs make up the predominant (70 percent or more) population in soybean fields. The big question, Herbert says, is green stink bug an economic threat to Virginia soybeans. Based on current research findings the answer is no.

While it would be easy for farmers to conclude from these results that green stink bugs aren’t a threat to Virginia soybeans, Herbert says he’s not so sure. “I’m not ready to trust these data. I’m not ready to say don’t spray for stink bugs. Our challenge is to continue on with this research until we know for sure these bugs aren’t a threat to yield and quality in soybeans,” Herbert adds.

“One top crop consultant, Stan Winslow, first brought this to our attention a few years back. In eastern North Carolina, they get a lot of stink bugs in soybeans. In some fields, you can walk out and see stink bugs on the plant. It would be hard to tell a grower in a situation like that to not spray.”

The good news is that green stink bugs are easy to manage with insecticides. Growers routinely kept numbers low while spraying organophosphate and pyrethroid insecticides for other, more economically damaging, insect pests.

Soybeans are the most variable crop grown in the upper Southeast. When considering stink bug damage, such factors as maturity group, seeding rate, determinant versus non-determinant, row width, plant date and on and on.

Cotton, by comparison, is grown on a fairly standard row planting, standard seeding rate, relatively few varieties. Corn, again, is much less variable than soybeans.

“Hopefully, analysis of our increased threshold test across Virginia, Maryland and Delaware will provide us some better information to answer the question of whether green stink bugs are a threat to soybeans in our region,” Herbert concludes.